Im thinning a yellow-brown mix and its acting strange. The pigment is separating and floating around in small clumps. Any suggestions?
Im using brush cleaner because i dont have any turpentine, but i would have thought its about the same stuff. What ratio of paint to thinner is good for air brushing this stuff as a thin coat to build up several layers of?

As far as I'm aware, brush cleaner is designed to break down (destroy) oil based paints and not just to dilute them (as per thinners).

Turps is also not thinners so I would strongly advise that you get yourself a good quality thinners for thinning and use the cleaner for cleaning.

Lee

I agree with Lee, I think your problem is using "Brush Cleaner" instead of "Thinner".
"Brush Cleaner" is a much stronger solvent than thinner. For Humbrols, I prefer Humbrol's own thinner. It a very mild thinner, and has a better smell them most others.